1. Field of the Invention
The present invention relates to a communication control apparatus, particularly to a communication control apparatus used for performing communication among a plurality of electronic devices mounted on a vehicle.
2. Description of the Related Art
Typically, a plurality of electronic devices are mounted on a vehicle and the electronic devices are connected with each other through a wired network. In the wired network, car body information and the like can be transmitted by using a communication control apparatus mounted on the vehicle. The communication control apparatus has a circuit dedicated for disconnecting an electronic device from the wired network when something wrong occurs in the electronic device. This circuit prevents a bad effect on other electronic devices when one electronic device is broken down in the vehicle, so that communication can be continued by normal electronic devices.
FIG. 4 shows an example of the above-described circuit. FIG. 4 shows a circuit configuration of a known communication control apparatus. This circuit mainly includes a control unit 31 configured of a micro computer or the like, a trouble detecting unit 32 for detecting a trouble in the control unit 31, and a communication control unit 33 that interrupts communication when a trouble is detected, that is, controls communication by disconnecting an electronic device. The trouble detecting unit 32 includes a watchdog circuit 321 for observing signals from the control circuit 31 and a mono multi unit 322 for generating square waves in response to signals from the watchdog circuit 321.
In the communication control apparatus shown in FIG. 4, a pulse signal (clock pulses: signal “a” in FIG. 4) of predetermined intervals is output from the control unit 31 to the watchdog circuit 321 when the control unit 31 normally operates. If the control unit 31 breaks down (or has a trouble) and no pulse signal is output (right side of a broken line of signal “a” in FIG. 4), the watchdog circuit 321 cannot detect a pulse signal and outputs reset pulses (signal “b” in FIG. 4) to the mono multi unit 322. After receiving the reset pulses, the mono multi unit 322 generates a square wave (signal “c” in FIG. 4) whose output time is longer than that of the reset pulse. The communication control unit 33 receives the square wave and then controls communication by disconnecting the electronic device (signal “d” in FIG. 4). In this way, when a failure or trouble occurs in a specific electronic device, the electronic device can be disconnected from the wired network. (Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-240969)
In the configuration shown in FIG. 4, however, if noise occurs between the watchdog circuit 321 and the mono multi unit 322, the mono multi unit 322 outputs the noise while amplifying it onto a square wave having a long output time even when the noise has low energy. Therefore, even when the control unit 31 has neither a failure nor trouble, the communication control unit 33 controls communication by disconnecting an electronic device in response to the square wave.